Control circuit for a liquid vaporizing tool

ABSTRACT

A control circuit for controlling the temperature of a liquid vaporizing tool to a temperature required to vaporize the liquid. A low voltage is applied to heating elements on the tool to maintain the tool at the required temperature to vaporize the liquid during standby operation when there is no liquid being supplied to the tool. A high voltage is applied to the heating elements during operation of the tool, when liquid is being supplied thereto, so as to accommodate the increased heat input requirement caused by the vaporization of the liquid to maintain the tool at the vaporizing temperature. A thermal switch located on the tool opens or closes the respective voltage supply circuits to maintain the tool at the required vaporization temperature. In addition, to compensate for the thermal lag in the tool and to accommodate the sudden high heat demand caused by liquid vaporization when the liquid is initially supplied to the tool, the thermal switch is bypassed and the high voltage is applied to the heating elements for a specified time duration in anticipation of the thermal lag.

United States Patent Mikkola 1541 CONTROL CIRCUIT FOR A LIQUIDVAPORIZING TOOL [72] Inventor: John E. Mikkola, Ferndale, Mich.

[731 Assignee: General Motors Corporation,

Detroit, Mich.

[22] Filed: March 4, I971 [21] Appl. No.: 120,854

[52] US. Cl. ..219/272, 219/482, 307/86 [51] Int. Cl ..F22b [58] Fieldof Search ..219/272, 482, 377, 328, 329, 219/364; 307/80, 85, 86

[56] References Cited UNITED STATES PATENTS 2,649,530 8/1953 Dietz..219/482 X 3,365,567 1/1968 Smith et al ..219/272 3,486,081 12/1969Kanbar ..219/364 UX 3,623,050 11/1971 Kojima ..307/86 X FOREIGN PATENTSOR APPLICATIONS 882,878 ll/l961 Great Britain ..219/272 1 1 3,686,475 11 Aug. 22, 1972 Primary Examiner-D. F. Duggan Attorney-C. R. Meland andTim G. Jagodzinski 57 I ABSTRACT A control circuit for controlling thetemperature of a liquid vaporizing tool to a temperature required tovaporize the liquid. A low voltage is applied to heating elements on thetool to maintain the tool at the required temperature to vaporize theliquid during standby operation when there is no liquid being suppliedto the tool. A high voltage is applied to the heating elements duringoperation of the tool, when liquid is being supplied thereto, so as toaccommodate the increased heat input requirement caused by thevaporization of the liquid to maintain the tool at the vaporizingtemperature. A thermal switch located on the tool opens or closes therespective voltage supply circuits to maintain the tool at the requiredvaporization temperature. In addition, to compensate for the thermal lagin the tool and to accommodate the sudden high heat demand caused byliquid vaporization when the liquid is initially supplied to the tool,the thermal switch is bypassed and the high voltage is applied to theheating elements for a specified time duration in anticipation of thethermal lag.

4 Claims, 2 Drawing Figures Patented Aug. 22, 1972 3,686,475

CONTROL CIRCUIT SOLVENT UM REGULATOR p p SUPPLY RI .74 7 TO AC POWER HISUPPLY I NVINTOR.

BY 6/5/72: 5. ZYZ'Z'XkO/Q dZWeM ATTORNEY CONTROL CIRCUIT FOR A LIQUIDVAPORIZING TOOL This invention relates to a control circuit forcontrolling the temperature of a liquid vaporizing tool. Morespecifically, this invention relates to. a control circuit whichsupplies two levels of electrical power for maintaining the temperatureof thevaporizing tool in standby and in-use operations respectively andfor providing a timed override of the heat controlling element in thevaporizing tool upon start up in anticipation of the thermal lag in thetool.

In liquid vaporizing systems, the liquid vaporizing tool is heated to atemperature sufficient to vaporize the liquid supplied thereto. In knownsystems of this type, the output of a power supply is supplied to heating elements on the vaporizing tool through a thermal switch whichperiodically opens and closes in response to the temperature of the toolto maintain the vaporizing tool at the temperature required to vaporizethe liquid. When the output potential of the power supply is of such amagnitude so as to be able to maintain the vaporizing tool at atemperature sufficient to vaporize the liquid when the vaporizing toolis in operation, this potential, when the vaporizing tool is in standbyoperation in which liquid is not being supplied to the tool but the toolis maintained at'the required vaporizing temperature, far exceeds themagnitude necessary in order to maintain the vaporizing tool at therequired temperature. Consequently, during standby operation when it isdesired to maintain the tool at the desired temperature, the voltageapplied to the heating elements far exceeds that actuallyrequired tomaintain the gun at that temperature with the resulting excessiveoperation of the thermal switch. The consequence of this form ofoperation is a decreased operating life of the heater, tool and controlcircuit.

In addition, when the priorliquid vaporizing systems are switched fromstandby operation to their in-use operation, the sudden heat dissipationcaused by the vaporizing liquid lowers the temperature "of thevaporizing tool to a point at which a portion of the vapor condenses andis ejected from the tool in the form of liquid before the thermal switchsenses the decrease in temperature and supplies power to the heatingelements to again bring the vaporizing tool up to the requiredtemperature. This is a consequence of the thermal lag in the tool.

It is the general object of this invention to provide a control circuitfor controlling the temperature of a vaporizing tool in a liquidvaporizing apparatus.

It is another object of this invention to provide a control circuit fora liquid vaporizing tool which will improve the operating life of thecircuit and tool. It is another object of this invention to provide acontrol circuit for a liquid vaporizing tool in which heating elementsin the tool are supplied with a low voltage for standby operation andsupplied with a high voltage for in-use operation.

It is another object of this invention to provide a control circuit toprevent condensation of the liquid in the vaporizing tool during startup caused by the thermal lag of the tool.

These and other objects of this invention are accomplished by a circuitwhich is responsive to a sensed temperature of the vaporizing tool belowthe temperature required to vaporize the liquid when the liquid is notbeing supplied to the tool (standby operation) to supply a low voltageto heating elements on the tool. When the tool is switched from standbyto in-use operation, the circuit supplies a high voltage to the heatingelements for a time period independent from the sensed temperature ofthe tool to compensate for the thermal lag in the tool and thereaftersupplies the high voltage to the heating elements when the sensedtemperature of the tool is below the required temperature.

The invention may be best understood by reference to the followingdescription of a preferred embodiment and the drawings in which:

FIG. 1 is a schematic diagram of the liquid vaporizing system using thecontrol circuit of this invention;and

FIG. 2 is a circuit diagram of the control circuit used in the preferredembodiment of this invention.

The preferred embodiment is hereinafter described with reference to asolvent vaporizing system in which a paint solvent is vaporized in avaporizing tool and the resulting vapor is directed at a previouslypainted surface to reflow the paint. It is understood that the system isnot limited to the vaporization of paint solvents but may be used in thevaporization of any liquid.

Referring to FIG. 1, paint solvent from a solvent supply 10 is suppliedto a vaporizing tool 12 by a pump 13 through a pressure regulator 16 anda supply line 18. A trigger 14 on the vaporizing tool 12 controls anonoff valve 19 which prevents the flow of solvent through the vaporizingtool 12 when the trigger 14 is in its neutral position and allows theflow of fluid through the vaporizing tool 12 when operated. Themechanical interconnections between the trigger l4 and the valve 19 canbe made in any manner known to those skilled in the art. To vaporize thesolvent flowing into the vaporizing tool 12, heating elements aremounted on the vaporizing tool 12, and power is supplied thereto to heatthe vaporizing tool 12 to a temperature required to vaporize the solventand to maintain the vaporizing tool 12 at that temperature. A controlcircuit 20 in conjunction with a thermal switch mounted on thevaporizing tool 12 controls the power input to the heating elements tomaintain the desired temperature.

When the trigger 14 is actuated, solvent is supplied to the heatedvaporizing tool 12 and is vaporized thereby, the solvent vapor beingdirected by the vaporizing tool 12 at the paint surface to be reflowed.

Referring to FIG. 2, there is shown the control circuit 20 of FIG. 1.Basically, the control circuit is comprised of a power supply circuit 22and a circuit 24 for controlling the power supply circuit 22.

The power supply circuit 22 is comprised of a transformer 26 having aprimary winding 28 and a secondary winding 30. The primary winding 28 isconnected to any conventional AC power supply (not shown). The secondarywinding 30 has a common'terminal 32, a high voltage output terminal 34and a low voltage terminal 36. The high voltage terminal 34 is connectedto one side of a pair of heating elements 38 through a set of normallyopen relay contacts R1 The low voltage terminal 36 is connected to thesame side of the pair of heating elements 38 through a set of normallyopen relay contacts H1. The common terminal 32 is connected to theremaining side of the heating elements 38. The heating elements 38 areof a conventional form of resistive heating elements well known in theart.

As previously mentioned with reference to FIG. 1, the heating elements38 are mounted on the vaporizing tool 12 in FIG. 1 and are operative toheat the tool 12 upon the closing of either the set of normally openrelay contacts R1 or H1. When the set of normally open contacts H1 areclosed, low voltage from the low voltage terminal 36 is supplied to theheating elements 38 to provide for a low level heat input to thevaporizing tool 12. When the set of normally open contacts R1 areclosed, high voltage from the high voltage terminal 34 is supplied tothe heating elements 38 to provide a high heat input to the vaporizingtool 12.

To control the heat input to the vaporizing tool 12, the circuit 24controls the closure of the sets of normally open relay contacts R1 andH1 in a manner so as to maintain the temperature of the vaporizing tool12 at a temperature required to vaporize the liquid solvent suppliedthereto.

Power is supplied to the circuit 24 from the output terminals 40 and 42of the secondary winding 44 of a transformer 46. The transformer 46 alsoincludes a primary winding 48 connected across the AC power supply.

The circuit 24 includes three relay coils R, H and T.

The relay coil R controls the set of normally open relay contacts R1 inthe power supply circuit 22, the relay coil H controls the set ofnormally open relay contacts H1 in the power supply circuit 22, and'therelay coil T controls the sets of normally open relay contacts T1 andT2, a set of normally closed relay contacts T3 and a set of normallyclosed time delay contacts T4. The sets of relay contacts R1, H1, T1, T2and T3 are all instantaneously actuated in response to the energizationor de-energization of the respective relay coils R, H and T. The set ofnormally closed time delay contacts T4 'remain closed upon theenergization of the relay coil T for a predetermined time period afterwhich they open. This time delay is selected as a function of thethermal lag in the vaporizing tool 12 for reasons as will subsequentlybe described.

One side of the relay coil R is connected directly to the outputterminal 42. The remaining side of the relay coil R is connected to theoutput terminal 40 through the set of normally open relay contacts Tl, athermal switch 50 and an on-off switch 52 and is also connected to theterminal 40 through the set of normally open relay contacts T1, the setof normally closed time delay contacts T4, the set of normally openrelay contacts T2 and the on-off switch 52. One side of the relay coil His connected directly to the output terminal 42. The remaining side ofthe relay coil H is connected to the output terminal 40 through the setof normally closed contacts T3, the thermal switch 50 and the on-offswitch 52 and is also connected to the terminal 40 through the set ofnormally closed contacts T3, the set of normally closed time delaycontacts T4, the set of normally open contacts T2 and the on-off switch52. One side of the relay coil T is connected directly to the outputterminal 42 and the remaining side is connected to the output terminal40 through a valve switch 54 and the on-off switch 52.

As previously indicated with reference to FIG. 1, the thermal switch 50is mounted on the vaporizing tool l2 to monitor 'thetemperature thereof.This switch may be any one of the well known forms of thermal switchesand is adapted so as to open when the temperature of the vaporizing tool12 is equal to or exceeds the temperature required to vaporize thesolvent.

The valve switch 54 is mounted on the vaporizing tool 12 and is adaptedto be opened when the trigger 14 is in its neutral position and to beclosed when the trigger 14 is in its actuated position.

Referring to FIGS. 1 and 2, when it is desired to place the solventvaporizing system in standby operation, the on-off switch 52 in thecircuit 24 of FIG. 2 is closed. Upon the closing of the 'on-off switch52. the relay coil H is energized through the thermal switch 50 and theset of normally closed contacts T3. Energization of the relay coil- Hcloses the set of normally open contacts H1 in the power supply circuit22 to supply low voltage from the low voltage terminal 36 of thesecondary winding 30 to the heating elements 38. This low voltage issufficient to cause the heating elements 38 to heat the vaporizing tool12 to the temperature required to vaporize the solvent. When the temperature of the vaporizing tool 12 reaches the required temperature, thethermal switch 50 will periodically open and close in response to thetemperature of the vaporizing tool 12 to cause the relay coil H toperiodically open and close the set of normally open contacts H1 tomaintain the vaporizing tool 12 at the temperature required to vaporizethe solvent. The vaporizing tool is now in a standby condition.

When it is desired to reflow a previously painted surface, the trigger14 of the vaporizing tool 12 is actuated to open the valve 19 to supplysolvent to the vaporizing tool 12 and to close the normally open contact54.

Upon the actuation of the trigger 14, the vaporization of the solventsupplied to the vaporizing tool 12 causes a sudden increase in heatdissipation from the vaporizing tool 12. Due to a thermal lag in thevaporizing tool, the vaporizing tool 12 may cool to a temperature atwhich condensation of the solvent vapor and an incomplete vaporizationof the solvent will occur before the thermal switch 50 can detect such adecrease in temperature. In addition, the input to the heating elements38 from the low voltage terminal 36 is insufficient to maintain thetemperature of the tool 12 when solvent is being vaporized. Tocompensate for the thermal lag of the vaporizing tool 12 and to providethe additional heat input to compensate for the increased heatdissipation due to solvent vaporization, a high voltage is supplied tothe heating elements 38 and the thermal switch 50 is bypassed for a timeperiod sufficient to overcome the thermal lag. This is accomplished uponthe actuation of the trigger 14 and the resulting closure of thecontacts 54 and energization of the relay coil T. Energization of therelay coil T opens the set of normally closed contacts T3 to de-energizethe relay coil H. The resulting opening of the set of normally opencontacts H1 disconnects the low voltage output terminal 36 from theheating elements 38. Simultaneously therewith, the sets of normally opencontacts T1 and T2 are closed to energize the relay coil R through theset of normally closed time delay contacts T4 independent of thetemperature of the vaporizing tool 12 as sensed by the thermal switch 50for the duration of the time delay of the contacts T4. During" this timeperiod, the set of normally open contacts R1 are closed to supply highvoltage from the high voltage output terminal 34 to the heating elements38. Consequently, the heating elements 38 supply a high heat input tothe vaporizing tool 12 independent of the behavior of the thermal switch50. Since the time delay of the set of normally closed time delaycontacts T4 is chosen as a function of the thermal lag of the vaporizingtool 12, the vaporizing gun 12 is maintained at the temperature requiredto vaporize the solvent after the actuation of the trigger 14. Afterthis time delay, the set of normally closed time delay contacts T4 openand thereafter the relay coil R is energized through the thermal switch50. The set of normally open contacts R1 are, therefore, periodicallyopened and closed in response to the cyclic de-energization andenergization of the relay coil R by the thermal switch 50 toperiodically supply high voltage from the high voltage terminal 34 tothe heating elements to maintain the vaporizing tool 12 at the requiredtemperature to vaporize the solvent, the high voltage being required tomaintain that temperature due to the heat dissipation caused by solventvaporization.

When the trigger 14 is released and allowed to return to its neutralposition, the valve 19 is closed to prevent solvent from flowing throughthe vaporizing tool 12 and the valve switch 55 is opened to de-energizethe relay coil T to again put the circuit 24 in standby condition aspreviously described. 7

The detailed description of the preferred embodiment of the inventionfor the purpose of explaining the principles thereof is not to beconsidered as limiting or restricting the invention, since manymodifications may be made by the exercise of skill in the art withoutdeparting from the scope of the-invention.

I claim:

1. A control apparatus for controlling the temperature of a liquidvaporizing tool in a liquid vaporizing system which includes the tool,liquid supply means for supplying liquid to the tool and a valve havingopen and closed positions for selectively allowing or preventing,respectively, the flow of the liquid to the tool, the control apparatuscomprising heating elements mounted on the tool, the elements beingresponsive to voltage applied thereto for heating the tool; a thermal,switch mounted on the tool, said switch having a first state when thetemperature of the tool is below the temperature required to vaporizethe liquid and a second state when the temperature of the tool is at orabove said temperature; a first voltage supply having a pair of outputterminals; a second voltage supply having a pair of output terminals,the voltage output of the second voltage supply being greater than thevoltage output of the first voltage supply; first circuit meansresponsive to valve position and to the state of the thermal switch forconnecting the pair of output terminals of the first voltage supplyacross the heating elements when the valve is in its closed position andthe thermal switch is in its first state; second circuit meansresponsive to valve position and to the state of the thermal switch forcon necting the pair of output terminals of the second voltage supplyacross the heating elements when the valve is in its open position andthe thermal switch is in its first state, whereby the first voltagesupply supplies a voltage to the heating elements to maintain the toolat the temperature required to vaporize the liquid when the valve isclosed and the second voltage supply supplies a greater voltage to theheating elements to maintain the tool at said temperature when the valveis open, the voltage required to maintain the tool at said temperaturebeing greater when the valve is open than when the valve is closed dueto heat dissipation caused by the liquid vaporization.

2. A control apparatus for controlling the temperature of a liquidvaporizing tool in a liquid vaporizing system which includes the tool,liquid supply means for supplying liquid to the tool and a valve havingopen and closed positions for selectively allowing or preventing,respectively, the flow of the liquid to the tool, the control apparatuscomprising heating elements mounted on the tool, the heating elementsbeing responsive to voltage supplied thereto for heating the tool; athermal switch mounted on the tool, said switch having a first statewhen the temperature of the tool is below the temperature required tovaporize the liquid and a second state when the temperature of the toolis at or above said temperature; a first voltage supply having a pair ofoutput terminals; a second voltage supply having a pair of outputterminals, the voltage output of the second voltage supply being greaterthan the voltage output of the first voltage supply; first switch meansoperative when energized to connect the output terminals of the firstvoltage supply across the heating elements; second switch meansoperative when energized to connect the output terminals of the secondvoltage supply across the heating elements; first circuit means forenergizing the first switch means when the thermal switch is in itsfirst state and the valve is in its closed position, the tool beingmaintained at the temperature required to vaporize the liquid when thevalve is closed by the voltage supplied to the heating elements from thefirst voltage supply; and second circuit means for energizing the secondswitch means when the thermal switch is in its first state and the valveis in its open position, the tool being maintained at the temperaturerequired to vaporize the liquid when the valve is open by the increasedvoltage supplied to the heating elements from the second voltage supply,the increased voltage being required when the valve is open due to heatdissipation caused by the liquid vaporization.

3. A control apparatus for controlling the temperature of a liquidvaporizing tool in a liquid vaporizing system which includes the tool,liquid supply means for supplying liquid to the tool and a valve havingopen and closed positions for selectively allowing or preventing,respectively, a flow of the liquid to the tool, the control apparatuscomprising heating elements mounted on the tool, the heating elementsbeing responsive to voltage supplied thereto for heating the tool; athermal switch mounted on the tool, said switch having a first statewhen the temperature of the tool is below the temperature required tovaporize the liquid and a second state when the temperature of the toolis at or above said temperature; a first voltage supply having a pair ofoutput terminals; a second voltage supply having a pair of outputterminals, the voltage output of the second voltage supply being greaterthan the voltage output of the first voltage supply; first switch meansoperative when energized to connect the output terminals of the firstvoltage supply across the heating elements; second switch meansoperative when energized to connect the output terminals of the secondvoltage supply across the heating elements; first circuit meansincluding the thermal switch for energizing the first switch means whenthe valve is in its closed position and the thermal switch is in itsfirst state, the tool being maintained at the required temperature tovaporize the liquid when the valve is closed by the voltage supplied tothe heating elements from the first voltage supply; second circuit meansincluding the thermal switch for energizing the second switch means whenthe valve is in its open position and the thermal switch is in its firststate, the tool being maintained at the temperature required to vaporizethe liquid when the valve is open by the increased voltage supplied tothe heating elements from the second voltage supply, the increasedvoltage being required when the valve is open due to heat dissipationcaused by the liquid vaporization; and means for energizing the secondswitch means independent of the state of the thermal switch for apredetermined time period after the valve is shifted from its closedposition to its open position so as to supply increased voltage from thesecond voltage supply to the heating elements for the predetermined timeperiod independent of the thermal switch to compensate for the thermallag in the tool following the sudden increased heat requirement tomaintain the tool at the temperature required to vaporize the liquidcaused by the vaporization of the liquid.

4. A control apparatus for controlling the temperature of a liquidvaporizing tool in a liquid vaporizing system which includes the tool,liquid supply means for supplying liquid to the tool and a valve havingopen and closed positions for selectively allowing or preventing,respectively, a flow of the liquid to the tool, the control apparatuscomprising heating elements mounted on the tool, the heating elementsbeing responsive to voltage supplied thereto for heating the tool; athermal switch mounted on the tool, said switch having a first statewhen the temperature of the tool is below the temperature required tovaporize the liquid and a second state when the temperature of the toolis at or above said temperature; a'voltage supply having a pair ofoutput terminals; circuit means including the thermal switch forconnecting the output terminals of the voltage supply across the heatingelements when the thermal switch is in its first state, the tool beingmaintained at the required temperature to vaporize the liquid by thevoltage applied to the heating elements from the voltage supply; andmeans forconnecting the output terminals of the voltage supply acrossthe heating elements independent of the state of the thermal switch fora predetermined time period after the valve is shifted from its closedposition to its open position so as to apply the output of the voltagesupply continuously to the heating elements for the predetermined timeperiod independent of the thermal switch to compensate for the thermallag in the tool following the sudden increased heat requirement tomaintain the tool at the temperature required to vaporize the liquidcaused by the vaporization of the liquid.

1. A control apparatus for controlling the temperature of a liquidvaporizing tool in a liquid vaporizing system which includes the tool,liquid supply means for supplying liquid to the tool and a valve havingopen and closed positions for selectively allowing or preventing,respectively, the flow of the liquid to the tool, the control apparatuscomprising heating elements mounted on the tool, the elements beingresponsive to voltage applied thereto for heating the tool; a thermalswitch mounted on the tool, said switch having a first state when thetemperature of the tool is below the temperature required to vaporizethe liquid and a second state when the temperature of the tool is at orabove said temperature; a first voltage supply having a pair of outputterminals; a second voltage supply having a pair of output terminals,the voltage output of the second voltage supply being greater than thevoltage output of the first voltage supply; first circuit meansresponsive to valve position and to the state of the thermal switch forconnecting the pair of output terminals of the first voltage supplyacross the heating elements when the valve is in its closed position andthe thermal switch is in its first state; second circuit meansresponsive to valve position and to the state of the thermal switch forconnecting the pair of output terminals of the second voltage supplyacross the heating elements when the valve is in its open position andthe thermal switch is in its first state, whereby the first voltagesupply supplies a voltage to the heating elements to maintain the toolat the temperature required to vaporize the liquid when the valve isclosed and the second voltage supply supplies a greater voltage to theheating elements to maintain the tool at said temperature when the valveis open, the voltage required to maintain the tool at said temperaturebeing greater when the valve is open than when the valve is closed dueto heat dissipation caused by the liquid vaporization.
 2. A controlapparatus for controlling the temperature of a liquid vaporizing tool ina liquid vaporizing system which includes the tool, liquid supply meansfor supplying liquid to the tool and a valve having open and closedpositions for selectively allowing or preventing, respectively, the flowof the liquid to the tool, the control apparatus comprising heatingelements mounted on the tool, the heating elements being responsive tovoltage supplied thereto for heating the tool; a thermal switch mountedon the tool, said switch having a first state when the temperature ofthe tool is below the temperature required to vaporize the liquid and asecond state when the temperature of the tool is at or above saidtemperature; a first voltage supply having a pair of output terminals; asecond voltage supply having a pair of output teRminals, the voltageoutput of the second voltage supply being greater than the voltageoutput of the first voltage supply; first switch means operative whenenergized to connect the output terminals of the first voltage supplyacross the heating elements; second switch means operative whenenergized to connect the output terminals of the second voltage supplyacross the heating elements; first circuit means for energizing thefirst switch means when the thermal switch is in its first state and thevalve is in its closed position, the tool being maintained at thetemperature required to vaporize the liquid when the valve is closed bythe voltage supplied to the heating elements from the first voltagesupply; and second circuit means for energizing the second switch meanswhen the thermal switch is in its first state and the valve is in itsopen position, the tool being maintained at the temperature required tovaporize the liquid when the valve is open by the increased voltagesupplied to the heating elements from the second voltage supply, theincreased voltage being required when the valve is open due to heatdissipation caused by the liquid vaporization.
 3. A control apparatusfor controlling the temperature of a liquid vaporizing tool in a liquidvaporizing system which includes the tool, liquid supply means forsupplying liquid to the tool and a valve having open and closedpositions for selectively allowing or preventing, respectively, a flowof the liquid to the tool, the control apparatus comprising heatingelements mounted on the tool, the heating elements being responsive tovoltage supplied thereto for heating the tool; a thermal switch mountedon the tool, said switch having a first state when the temperature ofthe tool is below the temperature required to vaporize the liquid and asecond state when the temperature of the tool is at or above saidtemperature; a first voltage supply having a pair of output terminals; asecond voltage supply having a pair of output terminals, the voltageoutput of the second voltage supply being greater than the voltageoutput of the first voltage supply; first switch means operative whenenergized to connect the output terminals of the first voltage supplyacross the heating elements; second switch means operative whenenergized to connect the output terminals of the second voltage supplyacross the heating elements; first circuit means including the thermalswitch for energizing the first switch means when the valve is in itsclosed position and the thermal switch is in its first state, the toolbeing maintained at the required temperature to vaporize the liquid whenthe valve is closed by the voltage supplied to the heating elements fromthe first voltage supply; second circuit means including the thermalswitch for energizing the second switch means when the valve is in itsopen position and the thermal switch is in its first state, the toolbeing maintained at the temperature required to vaporize the liquid whenthe valve is open by the increased voltage supplied to the heatingelements from the second voltage supply, the increased voltage beingrequired when the valve is open due to heat dissipation caused by theliquid vaporization; and means for energizing the second switch meansindependent of the state of the thermal switch for a predetermined timeperiod after the valve is shifted from its closed position to its openposition so as to supply increased voltage from the second voltagesupply to the heating elements for the predetermined time periodindependent of the thermal switch to compensate for the thermal lag inthe tool following the sudden increased heat requirement to maintain thetool at the temperature required to vaporize the liquid caused by thevaporization of the liquid.
 4. A control apparatus for controlling thetemperature of a liquid vaporizing tool in a liquid vaporizing systemwhich includes the tool, liquid supply means for supplying liquid to thetool and a valve having open and closed positionS for selectivelyallowing or preventing, respectively, a flow of the liquid to the tool,the control apparatus comprising heating elements mounted on the tool,the heating elements being responsive to voltage supplied thereto forheating the tool; a thermal switch mounted on the tool, said switchhaving a first state when the temperature of the tool is below thetemperature required to vaporize the liquid and a second state when thetemperature of the tool is at or above said temperature; a voltagesupply having a pair of output terminals; circuit means including thethermal switch for connecting the output terminals of the voltage supplyacross the heating elements when the thermal switch is in its firststate, the tool being maintained at the required temperature to vaporizethe liquid by the voltage applied to the heating elements from thevoltage supply; and means for connecting the output terminals of thevoltage supply across the heating elements independent of the state ofthe thermal switch for a predetermined time period after the valve isshifted from its closed position to its open position so as to apply theoutput of the voltage supply continuously to the heating elements forthe predetermined time period independent of the thermal switch tocompensate for the thermal lag in the tool following the suddenincreased heat requirement to maintain the tool at the temperaturerequired to vaporize the liquid caused by the vaporization of theliquid.